1. Field of the Invention
The present invention relates to an image forming apparatus that outputs a toner image on a photosensitive drum onto an intermediate transfer medium or copy paper.
Priority is claimed on Japanese Patent Application No. 2009-244647, filed Oct. 23, 2009, and Japanese Patent Application No. 2010-039820, filed Feb. 25, 2010, the content of which is incorporated herein by reference.
2. Description of Related Art
In this type of image forming apparatus electrophotography is used. A charging unit preliminarily charges a photosensitive drum, and when an exposure section irradiates light onto the surface of the photosensitive drum, an electrostatic latent image is formed on this photosensitive drum surface. A developer supports a toner, and when a developing bias voltage is applied thereto, the toner becomes excited to adhere to the electrostatic latent image and a toner image is formed (i.e., developed) on the surface of the photosensitive drum. The visibilized toner image is then either transferred onto copy paper or is transferred onto copy paper via an intermediate transfer medium and is then fixed.
Here, a structure that irradiates static elimination light onto the surface of the photosensitive drum is known. This is because a memory image is generated unless the exposure history of the photosensitive drum is deleted. Therefore, a static elimination unit irradiates static elimination light before the surface of the photosensitive drum is charged, and thereby removes any charge (i.e., residual charge) remaining on the surface of the photosensitive drum after a transfer.
Note that there are cases when a minute amount of toner is not transferred onto the copy paper but remains on the surface of the photosensitive drum. This toner must be removed before the next image formation. Because of this, cleaning the surface of this photosensitive drum using a cleaning blade is also disclosed.
On the surface of a photosensitive drum, areas on the outer side of the paper feed width as seen from the direction of the axis of rotation of this photosensitive drum form a range where an image is not formed. This is because the transfer surface of the intermediate transfer medium does not come into contact here with the copy paper. In order to make it possible to also form a toner image at both the left and right ends of a sheet of copy paper, it is necessary to set the developing width wider than the paper feed width. In addition, the charging width is set wider than the developing width. This is because, if the charging width were to be narrower than the developing width, toner would continuously adhere to the outer side areas of this charging width.
Namely, by applying a developing bias voltage, toner (including external additives) is made to travel towards areas located on the outer side of the paper feed width which are also on the inner side of the developing width. However, because these areas correspond to the areas on the inner side of the charging width, the toner itself does not adhere to the surface of the photosensitive drum, but instead returns to the developer. In contrast to this, the external additives themselves become separated from the toner and are left behind on the relevant areas. Thereafter, these external additives alone are transferred from the surface of the photosensitive drum onto the transfer surface of the intermediate transfer medium, and become accumulated on the edge of the cleaning blade.
Here, a state in which only external additives have been transferred onto the transverse surface of the former is one that it is difficult for the cleaning apparatus of the intermediate transfer medium to clean. In addition, although only slight, the difference in linear velocity between the intermediate transfer medium and the photosensitive drum is an additional factor in causing the aforementioned areas of the surface of the photosensitive drum with which this intermediate transfer medium comes into contact which are located on the outer side of the paper feed width and are also located on the inner side of the developing width to become abraded by the external additives. Moreover, a state in which the external additives have been transferred onto the cleaning blade of the latter is one that these areas are abraded by the external additives via the cleaning blade, thus the problem arises that the amount of wear on the photosensitive drum in these areas shortens the lifespan of the photosensitive drum.
Further, in a photosensitive drum which is made of, for example organic materials and thus is comparatively easily abraded, if it makes direct contact with a contact-charging type charging roller, adjacent edge portions of this charging roller deteriorate easily due to electrical discharge.
Specifically, if the shape of a charging roller is formed as a simple circular-cylinder shape with sharply outstanding edges, then the pressing force from the charging roller onto the photosensitive drum surface becomes concentrated in the edge portions of the charging roller, so that distortion is generated which causes areas slightly on the inward side of these edge portions to separate from the photosensitive drum surface.
In addition, in this manner, the pressing force becomes concentrated in the edge portions of the charging roller, the current flowing from the edge portions to the photosensitive drum surface increases compared to that to areas on the inner side of the paper feed width and the deterioration of the surface of the photosensitive drum which is caused by the amount of electrical discharge (i.e., the electrical discharge energy) during charging accelerates. In particular, an excessive electrical discharge phenomenon occurs in areas somewhat on the inner side of these edge portions so that there is marked acceleration in the deterioration of the photosensitive drum surface which is caused by electrical discharge energy, and the problem arises that the deterioration of the photosensitive drum in these areas ends up causing the lifespan of the photosensitive drum to be shortened.
In order to solve this problem, even if the above described conventional technology is utilized, the only effect is that the amount of static elimination light irradiated onto the outer side areas of the paper feed width is increased compared with that onto the inner side areas of the paper feed width. Consequently, the amount of electrical discharge during charging increases resulting in a deterioration of the photosensitive drum surface, and the wear of the outer side areas of the paper feed width is further accelerated.